Hybrid automatic transmission

A hybrid automatic transmission of a hybrid electric vehicle mounting thereon a prime mover, first and second motor-generators and a drive train, and capable of steplessly varying a transmission gear ratio by controlling an operating state of each of the first and second motor-generators, comprising:

• a differential device having at least four rotating members and two degrees of freedom that determining rotating states of two members of the four rotating members enables rotating states of the other rotating members to be determined, a first member of the four rotating members serving as an input element connected to the prime mover, a second member serving as an output element connected to the drive train, a third member connected to the first motor-generator, and a fourth member connected to the second motor-generator;
• and
• a specified relationship among an inertia of a rotating system relating to the prime mover, an inertia of a rotating system relating to the output element, an inertia of a rotating system relating to the first motor-generator, an inertia of a rotating system relating to the second motor-generator, a first lever ratio of a distance between the input element and the first motor-generator to a distance between the input and output elements, and a second lever ratio of a distance between the output element and the second motor-generator to the distance between the input and output elements is determined so that a center of gravity of a lever on an alignment chart of the hybrid automatic transmission is laid out on the output element or between the output element and the second motor-generator.